Many optical projection systems include a transmissive or a reflective imager, also referred to as a light valve or light valve array, which imposes an image on a light beam. Transmissive light valves are typically translucent and allow light to pass through. Reflective imagers, on the other hand, reflectively modulate only selected portions of the input beam to form an image. Reflective imagers provide important advantages, as controlling circuitry may be placed behind the reflective surface and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness. New potentially inexpensive and compact liquid crystal display (LCD) projector configurations may become possible by the use of reflective liquid crystal microdisplays as the imager.
Many reflective LCD imagers, such as liquid crystal on silicon (LCoS) imagers rotate the polarization of incident light. In other words, polarized light is reflected by the imager either with its polarization state substantially unmodified for the darkest state, or with a degree of polarization rotation imparted to provide a desired grey scale. A 90° rotation provides the brightest state in these systems. Accordingly, a polarized light beam is generally used as the input beam for reflective LCD imagers. Some existing arrangements include a folded light path between a polarizing beamsplitter (PBS) and the imager, wherein the illuminating beam and the projected image reflected from the imager share the same physical space between the PBS and the imager. The PBS separates the incoming light from the polarization-rotated image light. Typically, a single imager may be used for forming a monochromatic image or for forming a color image through the sequential application of light in different color bands to the image, commonly referred to as “field sequential color”.
Multiple imagers may also be used for forming a color image, in which case the illuminating light is split into multiple beams of different color. An image is imposed on each of the beams individually, which are then recombined to form a full color image. A multiple imager system requires that the alignment is able to withstand mechanical and thermal stresses. It is desirable to use as much light generated by the light source as possible. It is also desirable for the projection engine to be inexpensive and to have reduced thermal and mechanical instabilities.